1. Field of the Invention
The invention relates to isocyanate-free foamable mixtures.
2. Description of the Related Art
Sprayable in-situ foams are used for filling hollow spaces especially in the building sector. Here, they are used, inter alia, for sealing joints, e.g. around windows and doors, and they act as excellent insulating materials and lead to good thermal insulation. Further applications are, for example, insulation of pipes or foaming of hollow spaces in various apparatuses and appliances.
All conventional in-situ foams are polyurethane foams (PU foams) which in the uncrosslinked state comprise prepolymers which have a high concentration of free isocyanate groups. These isocyanate groups are able to undergo addition reactions with suitable reactants even at room temperature, resulting in curing of the spray foam after application. The foam structure is generated by a volatile blowing agent being mixed into the still uncrosslinked raw material and/or by means of carbon dioxide formed by reaction of isocyanates with water. The foam is usually supplied from pressure cans and ejected under the intrinsic pressure of the blowing agent.
Compounds which are reacted with the isocyanates are alcohols having two or more OH groups, especially branched and unbranched polyols, or else water. The latter reacts with isocyanates to liberate carbon dioxide, as mentioned above, and form primary amines which can then add directly onto a further, not yet reacted isocyanate group. These reactions form urethane and urea units which, owing to their high polarity and their ability to form hydrogen bonds in the cured material, can form partially crystalline substructures and thus lead to foams having a high hardness, compressive strength and ultimate tensile strength.
Blowing agents used are usually gases which are condensable at a relatively low pressure and can thus be mixed in the liquid state into the prepolymer mixture without the spray cans having to be subjected to excessively high pressures. The prepolymer formulations further comprise additional additives such as foam stabilizers, emulsifiers, flame retardants, plasticizers and catalysts. The latter are usually organic tin(IV) compounds or tertiary amines. However, iron(III) complexes, for example, are also suitable here.
PU spray foams are produced both as one-component (1K) foams and two-component (2K) foams. The 1K foams cure exclusively by contact of the isocyanate-containing prepolymer mixture with atmospheric moisture. The carbon dioxide liberated during the curing reaction of the 1K foams can additionally aid foam formation. 2K foams consist of an isocyanate component and a polyol component which have to be intimately mixed with one another immediately before foaming and cure as a result of the reaction of the polyol with the isocyanates. An advantage of the 2K systems is an extremely short curing time of sometimes only a few minutes for complete curing to occur. However, they have the disadvantages that they require a more complicated pressure can with two chambers and are also significantly less comfortable to handle than the 1K systems.
The cured PU foams have, in particular, excellent mechanical and thermal insulation properties. Furthermore, they display very good adhesion to most substrates and have virtually unlimited stability under dry conditions in the absence of UV radiation. Further advantages are the nontoxic nature of the cured foams after all isocyanate units have quantitatively reacted, and their rapid curing and ease of handling. Owing to these properties, PU foams have been found to be very useful in industry.
However, PU spray foams have the critical disadvantage that the isocyanate groups can, owing to their high reactivity, also display extreme irritant and toxic properties. In addition, the amines which can be formed by reaction of monomeric diisocyanates with an excess of water are in many cases suspected of being carcinogenic. Such monomeric diisocyanates are present in addition to the isocyanate-terminated prepolymers in most spray foam formulations. For this reason, the uncrosslinked spray foam compositions are not without toxicological concerns before they are fully cured. Critical aspects here are direct contact of the prepolymer mixture with the skin and, in particular, a possible aerosol formation during application of the foam or vaporization of low molecular weight constituents, e.g. monomeric isocyanates. As a result, there is a risk of toxicologically unacceptable compounds being inhaled. In addition, isocyanates have a considerable allergenic potential and can, for example, trigger asthma attacks. These risks are increased by the fact that the PU spray foams are often used not by trained and practised users but by hobbyists and home handymen, so that correct handling cannot always be assumed.
As a result of the hazard potential of conventional PU foams and the associated compulsory labelling, the problem of seriously decreasing acceptance of the corresponding products by users has also occurred. In addition, empty or partly empty spray cans are classified as hazardous waste and have to be appropriately labelled and in some countries, e.g. Germany, even have to be made available for reuse by means of a costly recycling system.
To overcome these disadvantages, prepolymers for spray foams which contain only low concentrations, if any, of monomeric isocyanates have been described, for example in DE-A-43 03 848. However, such systems have the disadvantage that the prepolymers still have isocyanate groups, so that although such PU spray foams are better than conventional foams from a toxicological point of view, they are not unproblematical. Moreover, the acceptance and waste problems are not solved by such foam systems.
It would therefore be desired to have prepolymers which do not crosslink via isocyanate groups and are thus toxicologically acceptable available for the production of spray foams. These prepolymer mixtures should also make it possible to produce spray foams which in the cured state have similarly good properties, especially a comparable hardness, as conventional isocyanate-containing PU foams. In addition, one-component spray foam systems which cure exclusively by contact with atmospheric moisture have to be possible. They should at the same time display comparably trouble-free handling and processability including a high curing rate even at a low catalyst concentration. The latter is important especially because the organotin compounds generally used as catalysts are likewise associated with toxicological concerns. In addition, tin catalysts often also contain traces of highly toxic tributyltin derivatives. A prepolymer system which has such favorable curing properties that a tin catalyst can be entirely omitted would therefore be particularly advantageous.
Condensation-crosslinking silicone foams containing alkoxy-, acyloxy- or oximo-terminated silicone prepolymers have been described in the literature, e.g. in U.S. Pat. No. 6,020,389. Although such foamable mixtures are in principle suitable for producing 1K foams which at room temperature cure entirely as a result of the action of atmospheric moisture, such systems comprising purely silicone-containing prepolymers can only be used for producing elastic flexible to semirigid foams. They are not suitable for producing rigid, nonbrittle in-situ foams.
WO 00/04069 describes prepolymer formulations comprising alkoxysilane-terminated polyurethane prepolymers for producing rigid spray foams. These are polymers having a conventional polyurethane backbone which can be formed by reaction of customary diisocyanates with polyols. If an appropriate excess of diisocyanates is used in this first reaction step, isocyanate-terminated prepolymers are obtained. These can then be reacted with aminopropyltrimethoxysilane derivatives in a second reaction step to form the desired alkoxysilane-terminated polyurethane prepolymers. These prepolymers can condense with one another in the presence of water and a suitable catalyst with elimination of methanol and cure in this way. The water can be added as such or can come from contact with atmospheric moisture. Thus, both 1K and 2K foams can be produced using such a system.
The alkoxysilane-terminated polyurethane prepolymers described in WO 00/04069 have a spacer having 2-10 carbon atoms between the alkoxysilane chain ends and an adjacent urethane or urea unit. A disadvantage of this system is the only moderate reactivity of these silane-terminated polyurethane prepolymers. Relatively high concentrations of a tin catalyst are therefore required to achieve sufficiently rapid curing.